As the geometries of integrated circuits get smaller, it is more important that all aspects of processing be controlled. Manufacturing processes have been based on a "recipe" concept, that is, various procedures are followed to produce a desired effect with no exact knowledge of to what is actually occurring on the semiconductor wafer surface.
Rigorous attention to the "recipe" method has allowed semiconductor device manufacturers to progress to the present small feature size patterning, but with the requirement for tighter dimensional control on wafers with ever smaller geometries, the difficulty of controlling all the variables adequately in a particular process becomes far greater. Therefore, instead of attempting to control all the process variables to a high degree of accuracy, it would be much better to determine what constitutes the completion of the process, and use this information to control the ending of the cycle. This "endpoint detection" relieves the system of the requirements of extremely tight control of physical parameters and allows a more tolerant operating range.
Resist patterning techniques employed in the semiconductor lithographic process fundamental to integrated circuit manufacturing usually rely on a fluid dissolution step to remove photoresist polymer either made more soluble or left less resistant to dissolution by selective exposure to some type of photon irradiation or particle bombardment.
It is critically important to control this pattern developing dissolution carefully to achieve close dimensional control of pattern features, the tolerances of which affect yield and practicable design performance limits of semiconductor devices.
Present develop processes employ fixed developing times which are empirically predetermined to achieve the desired pattern dimensions, with every attempt being made to hold substrate, resist, and exposing and developing system parameters fixed at optimum values.
A more effective manufacturing process results when the rate and completion (endpoint) of material removed can be determined as each semiconductor wafer is being processed.
Accurate determination of endpoint can provide a basis for automatic adjustment of total develop time, which is composed of the time required to initially clear resist in the high solubility areas of the pattern plus predetermined additional develop time, for example 50% additional time past initial clearing.
The automatic develop time adjustment can largely compensate, as needed, for patterning process variations in such factors as: exposure system intensity and/or timing mechanism; resist thickness and sensitivity; substrate reflectance; develop solution chemical effectiveness, dispense rate, distribution, and temperature; chamber ventilation; wafer spin speed; and delay between exposure and development.
Monitoring of automatically determined developing times also provides an indication of the degree of control being achieved over the various process parameters and any significant drift of developing time can be used to alert technical personnel.
Monitoring changing thickness of transparent films by interpretation of optical interference occurring between film top and substrate reflections of a beam of monochromatic light is a method which has been effectively used in various material subtractive process in semiconductor fabrication, including resist developing in favorable circumstances.
The effectiveness of optical interference techniques for resist developing endpoint determination can be seriously degraded by processing considerations sometimes encountered in practice. In spin/spray developing processes, the spray can disperse the beam, and extraneous optical interference caused by varying developing fluid film thickness overlying the developing resist can also limit signal quality. Attempts to minimize spray density or fluid film thickness in order to enhance signal quality can degrade develop rate radial and angular uniformity. Low reflectivity of the wafer due to surface texture, transparent film optical interference integral to the semiconductor substrate, or semitransparent film absorption can reduce signal acuity. Also, a pattern with unfavorably low proportion of the resist area designed for removal presents little area changing in thickness such that little signal is obtainable.
Optical interference techniques require that there be optical sensor access over the wafer surface. Optimal sensor positioning may not be available in many present techniques due to the several fluid dispensing fixtures needed in the process.
Polymers employed in the formulation of photoresists typically exhibit high optical transmission at visible and near ultraviolet wavelengths and sharply lower transmission for shorter wavelengths, whereas developer fluids exhibit high transmission throughout this spectrum.
At the very short wavelengths employed in the present invention, optical transmission of the developer fluid is reduced greatly by even very low proportions of dissolved photoresist polymer. At intermediate ultraviolet wavelengths, greater proportions of dissolved polymer are required to cause sufficient absorption to greatly reduce optical transmission, such that sensitivity of the technique may be substantially affected by the wavelength chosen for analysis.
Experiment has shown that this high sensitivity to polymer presence of very short wavelength transmission can be employed to effectively analyze photoresist develop process effluent as a means of real-time monitoring of photoresist dissolution rate.